Chelating agents, also called complexing agents, are organic or inorganic compounds which can make heavy metal ions passivated through chelating with heavy metal ions and then forming a stable water-soluble complex. Chelating agents, containing electron donors to form coordinate bonds with heavy metal ions, are particularly effective in softening, descaling, rustproofing, stabilizing, synergism and so on. Therefore, chelating agents have been widely used in multiple technical fields such as water treatment, textile dying and papermaking, etc. According to their molecular structures, chelating agents can be roughly classified into groups including phosphate chelating agents, amino carboxylic acid chelating agents, hydroxy carboxylic acid chelating agents, organic phosphonic acid salt chelating agents and polycarboxylic acid chelating agents, etc.
Iminodisuccinate, a new-type amino carboxylic acid chelating agent, is extremely effective in chelating transition metal ions or alkali metal ions, particularly iron ions and copper ions, with its chelating power surpassing that of EDTA. Furthermore, in comparison to traditional common chelating agents such as phosphate, citric acid and EDTA, etc., iminodisuccinate chelating agents are readily biodegradable with excellent ecosystem compatibility, and have been considered as a true green chelating agent and gained wide market acceptance.
At present, LanXess in Germany has produced iminodisuccinate chelating agent in industrial scale and marketed it around the world. The production process of LanXess uses maleic anhydride and ammonia as the raw materials with pH of the reaction system later adjusted with the addition of sodium hydroxide and the reaction fulfilled under high temperature and high pressure, and eventually produces a mixed solution containing the iminodisuccinate chelating agent of over 70%. Unfortunately, high temperature and high pressure reaction adopted in this process not only poses huge potential safety hazards, but also puts stringent requirements in equipment selection, workshop design, space arrangement, production process management, etc. Thus, the production of iminodisuccinic acid salt with this process inherently is of high investment, high energy consumption, high cost and high risk. In addition, the use of ammonia in this process as the raw material will also unavoidably cause pollution to the environment and harm to the health of the operators.
Chinese patent CN1126733C also disclosed a method for preparing iminodisuccinate chelating agent. Still using maleic anhydride and ammonia as the raw materials, this method is claimed to be able to boost the yield of iminodisuccinate through controlling reaction system pH and enhance the conversion rate of maleic anhydride to over 97%. Unfortunately, the fulfilling of the reaction in this method also similarly needs the use of high temperature and high pressure, failing to address the issues of potential safety hazard and environmental pollution.
Wu Chang Yu, and etc. of Tianjin University (Wu Chang Yu, Wang Ya Quan and Li Jing, compound of green chelating agent iminodisuccinic acid salt and its chelation performance, chemical industry and project, March 2007, Vol. 24 No. 2) studied the preparation method of iminodisuccinic acid salt as well. They confirmed the optimal reaction conditions and speculated the reaction mechanism was that maleic acid and ammonia first created aspartic acid; then the aspartic acid further reacted with maleic acid and eventually generated iminodisuccinic acid. Furthermore, they briefly verified this proposed mechanism, and although it was confirmed that the reaction of aspartic acid and maleic acid as raw materials is able to produce iminodisuccinic acid salt, the reaction itself still requires the use of high pressure, and was not able to meet the requirements for industrialized production.